Hello everyone. I just released the first version of my plugin called Automatic Post Tagger. You can find it here: http://wordpress.org/extend/plugins/automatic-post-tagger
The purpose of this plugin is saving time to bloggers by tagging posts instead of them. After a post is published, the plugin looks for user-defined tags and their related words in the title, content and excerpt of the post. It can also assign tags to old posts as well, just by clicking one button.